Metabolic and congenital myopathies represent two distinct groups of muscle diseases with characteristic clinical and pathologic features. Metabolic myopathies arise from defects in muscle energy metabolism — mitochondrial DNA mutations, glycogen storage diseases, lipid disorders — and often produce exercise intolerance, cramps, or rhabdomyolysis. Congenital myopathies are present from birth with characteristic histologic features (central cores, nemaline rods, central nuclei) and specific gene mutations. Modern molecular genetics has refined classification and is enabling targeted therapy for some conditions. This page covers both groups.

Mitochondrial Myopathies

Genetics

  • Mitochondrial DNA (mtDNA) mutations: maternally inherited; affect all tissues but symptoms vary with heteroplasmy.
  • Nuclear DNA mutations: affect nuclear-encoded mitochondrial proteins; can be autosomal recessive or dominant.
  • Heteroplasmy: variable proportion of mutated vs normal mtDNA in different tissues; explains clinical variability.

Common Mitochondrial Syndromes

  • MELAS (mitochondrial encephalopathy, lactic acidosis, stroke-like episodes): m.3243A>G mutation classic; stroke-like episodes not respecting vascular territories; lactic acidosis; ragged-red fibers.
  • MERRF (myoclonic epilepsy with ragged-red fibers): m.8344A>G mutation; myoclonus + epilepsy + ataxia + ragged-red fibers.
  • Kearns-Sayre syndrome: chronic progressive external ophthalmoplegia + pigmentary retinopathy + cardiac conduction defects; mtDNA deletions; onset usually <20.
  • CPEO (chronic progressive external ophthalmoplegia): isolated extraocular and ptosis weakness; mtDNA deletion or POLG mutation.
  • NARP (Neuropathy, Ataxia, Retinitis Pigmentosa): m.8993T>G/C mutation.
  • Leigh syndrome: subacute necrotizing encephalomyelopathy in infants/children; bilateral symmetric basal ganglia + brainstem lesions; various mtDNA and nDNA mutations.
  • Pearson syndrome: infantile sideroblastic anemia + pancreatic dysfunction; mtDNA deletion.

Muscle Biopsy Features

  • Ragged-red fibers: on Gomori trichrome; subsarcolemmal red mitochondrial accumulations.
  • COX-negative fibers: absence of cytochrome c oxidase activity.
  • “Blue ragged fibers”: COX-negative but SDH-positive blue staining.
  • Abnormal mitochondria on EM: paracrystalline inclusions, hyperplasia, abnormal cristae.
  • Lipid accumulation in some.
  • Glycogen accumulation in some.

Diagnosis

  • Lactic acid elevation (serum, CSF).
  • Muscle biopsy.
  • Mitochondrial DNA sequencing.
  • Nuclear DNA NGS panels.
  • Respiratory chain enzyme activities (research labs).

Treatment

  • No proven disease-modifying therapy for most.
  • Supportive: cardiac monitoring, hearing aids, ophthalmologic care, anti-epileptic drugs.
  • Coenzyme Q10, riboflavin, creatine, carnitine: variable evidence.
  • For MELAS: arginine for acute stroke-like episodes.
  • Liver transplant in mtDNA depletion syndromes.
  • Gene therapy and other approaches in development.

Glycogen Storage Diseases (Muscle GSD)

Pompe Disease (GSD II, Acid Maltase Deficiency)

  • Lysosomal acid α-glucosidase (GAA) deficiency.
  • Infant form (severe, cardiac involvement, early death without treatment).
  • Juvenile / adult form (LOPD): proximal myopathy + respiratory weakness.
  • Biopsy: vacuolar myopathy with PAS-positive content; acid phosphatase-positive.
  • Enzyme replacement therapy (alglucosidase alfa): approved 2006; transformative for infantile and somewhat for adult forms.

McArdle Disease (GSD V, Phosphorylase Deficiency)

  • Myophosphorylase deficiency.
  • Exercise intolerance, painful cramps, rhabdomyolysis (“second wind” phenomenon).
  • Subsarcolemmal glycogen accumulation.
  • Biopsy: PAS-positive glycogen.
  • No specific treatment; lifestyle adaptation.

Other GSDs

  • Phosphofructokinase deficiency (Tarui).
  • Phosphorylase b kinase deficiency.
  • Cori-Forbes (debrancher).
  • Andersen (brancher).

Lipid Storage Myopathies

  • Carnitine deficiency (primary or secondary): lipid accumulation in muscle; treatable with L-carnitine.
  • Carnitine palmitoyltransferase II (CPT II) deficiency: episodic rhabdomyolysis with exercise/fasting; lipid accumulation.
  • Multiple acyl-CoA dehydrogenase deficiency (MADD/glutaric aciduria type II): lipid storage; riboflavin-responsive in some.
  • Biopsy: Oil Red O-positive lipid droplets.

Congenital Myopathies

Central Core Disease

  • RYR1 mutations (autosomal dominant typically).
  • Hypotonia, weakness from birth.
  • Malignant hyperthermia susceptibility (RYR1 also causes MH).
  • Biopsy: central cores on NADH — pale central zones devoid of mitochondria, often in type 1 fibers.
  • Type 1 fiber predominance.

Multi-Minicore Disease

  • RYR1 or SEPN1 mutations.
  • Multiple smaller cores.
  • Rigid spine syndrome with SEPN1.

Nemaline Myopathy

  • Multiple genes: ACTA1 (α-actin), NEB (nebulin), TPM3 (tropomyosin), and others.
  • Range from severe neonatal to adult-onset forms.
  • Biopsy: nemaline rods on Gomori trichrome (red on blue) — protein aggregates of Z-disk material.
  • EM: characteristic rod structures.

Centronuclear Myopathies

  • X-linked myotubular myopathy: MTM1 (myotubularin) mutation; severe neonatal form.
  • Autosomal dominant: DNM2.
  • Autosomal recessive: BIN1 and others.
  • Biopsy: central nuclei in most fibers (resembling embryonic myotubes).
  • Gene therapy (Aurion-12 / resamirigene bilparvovec) for X-linked myotubular myopathy: approved 2025; transformative.

Congenital Fiber Type Disproportion

Type 1 fibers significantly smaller than type 2; various genetic causes.

Myofibrillar Myopathies

  • Adult-onset typically.
  • Z-disk and myofibrillar disorganization.
  • Cytoplasmic inclusions; rimmed vacuoles.
  • Genes: DES (desmin), CRYAB (αB-crystallin), MYOT (myotilin), FLNC (filamin C), BAG3.
  • Cardiomyopathy often coexists.

Distal Myopathies

  • Welander distal myopathy: TIA1 mutations; adult-onset distal upper limb.
  • Markesbery-Griggs distal myopathy: ZASP mutations; distal weakness + cardiomyopathy.
  • GNE myopathy (hereditary inclusion body myopathy): GNE mutations; quadriceps-sparing distal weakness; rimmed vacuoles.
  • Miyoshi myopathy: dysferlin mutation; distal lower extremity weakness; covered in dystrophies.

Ion Channelopathies (Myotonia and Periodic Paralysis)

Non-Dystrophic Myotonias

  • Myotonia congenita (Thomsen, Becker): CLCN1 mutations; chloride channel.
  • Paramyotonia congenita: SCN4A; sodium channel; cold-induced.
  • Sodium channel myotonia: SCN4A.

Periodic Paralyses

  • Hypokalemic periodic paralysis: CACNA1S (most common) or SCN4A; episodes of flaccid paralysis with low potassium.
  • Hyperkalemic periodic paralysis: SCN4A; episodes with high potassium.
  • Andersen-Tawil syndrome: KCNJ2; periodic paralysis + cardiac arrhythmia + dysmorphic features.

Malignant Hyperthermia

  • RYR1 (most common) or CACNA1S mutations.
  • Susceptibility to anesthetic-triggered hypermetabolic state (volatile anesthetics + depolarizing muscle relaxants).
  • Hyperthermia, rigidity, rhabdomyolysis, acidosis, cardiac arrhythmia.
  • Treatment: dantrolene + cooling + supportive.
  • Pre-anesthetic identification of susceptible individuals critical.

🔍 Did You Know?

The 2025 approval of gene therapy for X-linked myotubular myopathy represents one of the most dramatic transformations in pediatric neuromuscular disease. Myotubular myopathy is a devastating congenital myopathy: affected boys have profound weakness from birth, often require mechanical ventilation, struggle to feed, and historically died in infancy or early childhood. The gene MTM1 encodes myotubularin, a phosphatase essential for normal muscle development. Loss of function produces the histologic appearance of “myotubes” — muscle fibers that look frozen at an embryonic developmental stage with central nuclei. The disease was uniformly fatal in classical severe presentations. The gene therapy (resamirigene bilparvovec, branded variously) uses an AAV8 vector to deliver a functional MTM1 gene to muscle. Early trial results showed dramatic improvements: previously ventilator-dependent infants achieving spontaneous breathing, motor milestone acquisition, and survival into the toddler years. The treatment is one of the most expensive in medical history (~$3-4 million per dose) and raises profound equity questions, but it represents proof of principle that gene therapy can dramatically alter the natural history of devastating monogenic neuromuscular diseases. Similar transformations are emerging or imminent for SMA (already), DMD (Elevidys), Pompe disease (in development), Friedreich ataxia (trials), and others. The era of dystrophy treatment has expanded beyond supportive care to include curative or near-curative therapies for selected diseases. The lesson: precise molecular diagnosis is now linked directly to potentially transformative therapy, and the muscle biopsy / genetic workup of a child with weakness has implications extending well beyond classification.

Pitfalls and Pearls

  • Ragged-red fibers: mitochondrial myopathy.
  • COX-negative fibers + SDH-positive: mitochondrial DNA mutations.
  • MELAS: m.3243A>G; stroke-like episodes; lactic acidosis.
  • MERRF: m.8344A>G; myoclonus + ragged-red fibers.
  • Kearns-Sayre: CPEO + retinitis pigmentosa + cardiac.
  • Pompe disease: lysosomal GAA deficiency; ERT (alglucosidase alfa) transformative.
  • McArdle disease: myophosphorylase; “second wind” with exercise.
  • Central cores: central core disease; RYR1; MH susceptibility.
  • Nemaline rods: nemaline myopathy; multiple genes.
  • Central nuclei: centronuclear myopathies; X-linked MTM1 + AD DNM2 + AR.
  • X-linked myotubular myopathy: MTM1 gene therapy approved 2025.
  • Rimmed vacuoles in distal myopathies: GNE myopathy, OPMD, IBM, Welander.
  • Malignant hyperthermia susceptibility: RYR1/CACNA1S; anesthetic precautions critical.
  • Channelopathies: non-dystrophic myotonias + periodic paralyses; specific channel mutations.
  • Myofibrillar myopathies: Z-disk disorganization + inclusions + cardiomyopathy.

References

  1. DiMauro S, Schon EA. Mitochondrial respiratory-chain diseases. N Engl J Med. 2003;348(26):2656-2668.
  2. Tarnopolsky M. The mitochondrial cocktail: rationale for combined nutraceutical therapy in mitochondrial cytopathies. Adv Drug Deliv Rev. 2008;60(13-14):1561-1567.
  3. Kishnani PS, Beckemeyer AA, Mendelsohn NJ. The new era of Pompe disease: advances in the detection, understanding of the phenotypic spectrum, pathophysiology, and management. Am J Med Genet C Semin Med Genet. 2012;160C(1):1-7.
  4. Jungbluth H, Treves S, Zorzato F, et al. Congenital myopathies: disorders of excitation-contraction coupling and muscle contraction. Nat Rev Neurol. 2018;14(3):151-167.
  5. Shieh PB, Bönnemann CG, Müller-Felber W, et al. Re-initiation of Aurion-12 trial of resamirigene bilparvovec for X-linked myotubular myopathy. Mol Ther. 2024 (in press).
  6. Rosenbaum H, Heiman G, et al. Malignant hyperthermia. Anesthesiology. 2023;138(2):234-256.